This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The present invention relates generally to the field of a refrigeration system primarily using CO2 as a refrigerant. The present invention relates more particularly to a CO2 refrigeration system using hot gas to provide defrost of evaporators.
Refrigeration systems typically operate at evaporator temperatures below the dewpoint of the air they are cooling and as such, frost is formed on the surface of the evaporator. Frost buildup on the evaporator reduces the heat transfer effectiveness of the heat exchanger and so the evaporators periodically go through a defrost cycle to remove the frost and return the heat transfer surface to a more optimal state.
Various methods to defrost evaporators are used and include time-off defrost, electric defrost, and hot gas defrost. Time-off defrost is considered a passive defrost system—the refrigeration system is turned off and the air moving across the evaporator provides the defrosting action—this method is generally only suitable for medium-temperature systems (evaporator temperatures greater than +15° F. or −10° C.). Electric and hot gas defrost, considered “active” or “forced” defrost methods, are typically suitable for both low- and medium-temperature refrigeration systems.
For electric defrost, an electric heater is located within or adjacent to the coil and heat flows into the evaporator either by conduction or convection by movement of air. This method requires additional wiring to be installed and additional electrical power to be used and many consider the extra installation and operating cost to be a drawback of this method.
For hot gas defrost, gas from the compressor discharge or other locations on the high-side of the system is typically passed through the coil either in a forward or reverse direction. The gas typically condenses to a liquid form inside the evaporator effectively heating the tubes from within—this is due primarily to the condensing temperature of the gas being above the freezing point of the frost (+32° F. or 0° C.). Hot gas defrost is generally considered less expensive to install and operate, but the pressure increase in the coil during the defrost cycle tends to raise concerns about long-term structural integrity (e.g. leak-tightness of the coil—it is believed that leaks can occur over time due to fatigue of the coil materials or joints).
Refrigeration systems utilizing carbon dioxide (“CO2” from here on) as the refrigerant are typically operated with electric defrost on the low-temperature system. Hot gas defrost has traditionally not been used in CO2 refrigeration systems because the pressure of the compressor discharge gas on the low-temperature side of the system is below the melting point of the frost (typical condensing temperature of approximately +20° F. or −7° C.) and therefore CO2 gas could only be desuperheated in the coil rather than condensing and a much smaller amount of heat would be available in the evaporator for defrosting purposes.
Accordingly, it would be desirable to provide a hot gas defrost system for a CO2 refrigeration system.